Decouple dual number field names from autodiff implementation

[samestep]

As of #51, the dual number type returned by the rose-autodiff crate puts the linear part before the nonlinear part because it knows that their field names will be "du" and "re" respectively, so they need to be in that order because our JS frontend requires struct field names to be alphabetized. We should find some way to relax this so that rose-autodiff doesn't need to know or care about field names at all.